text:
Does the avoidance of thrust reverser save fuel?
On 2017-06-17 I took an AirAsia Airbus A320-200neo from Hong Kong (VHHH) to Kuala Lumpur (WMKK).
The flight crew did not use reverse thrust for slowing down the plane.
Trying to find an explanation to this, I would like to know so much as possible about the use of thrust reversal by commercial flights, including low-cost airlines, which would probably like to use any feasible technique for saving fuel.

Possible duplicate of For large jets, what is the primary means of slowing down after landing?

Not a duplicate at all.

Is your belief that the reversers were not used based on the fact that you did not hear them or that you observed visually that they were not deployed? If it's just that you did not hear them, it may have been that the flying pilot chose to deploy them but not power up.

@Terry i didn't hear them nor felt their vibration.

Without further knowledge of the procedures/state of the (a) airport, (b) airline and (c) aircraft , I don't think any conclusive answer could be made. To throw some thoughts in:

All of Kuala Lumpurs's runways are over 4000 meters long, and braking an A320 (at landing weight) can easily be done without thrust reversers over that distance.
Depending where the gate was, they may simply have wanted use more of the runway to speed up the taxi, in which case excessive braking would have been undesirable. Most standard procedures call for a minimum of idle reverse thrust however.
The thrust reversers may have been inoperable, i.e. broken. You are allowed to fly an A320 in that state.

Update: Since you said you didn't sense any thrust reversers (vibration/sound, i.e. no visual confirmation), here's a video to show that they can be pretty quiet even if the doors are opened. If you follow the spiral painted on the spinner note the speed doesn't appear to change a lot even after the plane leaves the runway and the reverser doors are closed.
(Touchdown at 6:50)

I'd add to the list the fact that some captains prefer to deploy reversers but not power up. You still get much of the effect, especially with the bucket reversers (not the A320 type, though they do extend vanes into the airflow), but you avoid the noise and shaking. Also, some engines require a minimum time at idle thrust (or near idle thrust) for cooling purposes before shutdown, and avoiding powering up gets you to that time sooner.

in the video, despite it is quiet, one can hear the reverser. what i heard that time was: touchdown and absolutely no sound nor vibration of reverser. the runway 32L seems to be 4,096m and google maps shows the terminal at the end of the runway. the flight crew would porobably have had reasons to not power them up :D

There are two ways in which not using reverse will save fuel.  First is the obvious, in that when the engines are throttled up to fairly high RPM for the reverse thrust, that burns significantly more fuel than leaving them at idle.  So forgoing reverse thrust saves that fuel burn.
Beyond that, engines can be shut down after landing after they've had whatever cooling interval is recommended by the manufacturer, and in some cases, the power setting on approach is low enough that this time "counts" as cooling time, so if reverse thrust isn't used, then an engine (in the case of a twin) or a pair of engines (in the case of a 4-engine aircraft) can be shut down shortly after clearing the runway.  There is fuel savings here as well, not having to run the engine/s for the minutes required to give them the recommended cooling.
Additionally, not using reverse thrust can save on noise, which can be a "good neighbor" policy at urban airports at night.  Some airports & carriers will be more concerned about this than others.
Depending on where the aircraft will park, it may be advantageous to use the entire length of the runway (i.e. if you're landing to the west and you will park at the west end of the airport), and for a really long runway with ample room to slow down without reverse thrust or heavy braking, it can make sense to "coast" out to the end of the runway.
Three reasons that using reverse can be worthwhile:

Using more reverse means using the wheel brakes less.  They may be more expensive than the fuel & wear on the engines from using reverse thrust.
On a slick runway, the brakes may be ineffective at high speed, and the reverse thrust may be necessary to stop in the available runway. Some aircraft are certified in such a way that they have to be able to stop without the use of reverse thrust, so getting the TR's out is simply "bonus" stopping margin.  Others, however, are allowed to take credit for reverse thrust in the stopping calculations, so for those aircraft on sufficiently slick/short runways, using the TR's is vital to stopping within the available runway.
Finally, some operators require the pilots to use reverse thrust on every landing, even when it isn't needed.  This is to preserve the habit patterns & muscle memory so that they deploy the reversers every time, not just "when needed."  The logic here is that if you're making a decision at touchdown ("did I need them this landing?"), you may make the wrong decision when you really do need them, and the consequences in that case are very bad.  So rather than risk a bad decision, the airline policy is, don't make that decision -- always deploy them.

It also saves cycles on the TR, which can reduce maintenance costs.

Thrust reversers (T/Rs) are often not deployed due to mechanical failure; if a T/R is defective (won't retract, for example), depending on the airline, the mechanic can lock out the defective T/R. In my experience, this is by far the most common reason for not deploying a T/R.
It is far more expensive to replace brake assemblies than it is to pay for the additional fuel to stop the aircraft with a T/R.  Reduction of wear on brakes is one reason to always deploy the T/R whenever possible.
Source:  I was a Maintenance Technician for Delta Air Lines for 16 years.

oooo! Thanks so much!


text:
Were inclined runways ever under consideration?
A long time ago I read an article (in Popular Science, probably) of a plan for an airport runway constructed on a grade, so that one end was ten stories higher than the other. The idea was to save gas: at takeoff, the plane would be going downhill and get a speed boost from gravity. When landing, the plane would be going uphill, and take less engine power to stop because gravity is slowing it down.
Did anyone ever research or actually test this plan? Does it seem practical?

Well, you have Lukla Airport in Nepal with a 11% grade at 1,700 feet long which is like that because it is in the mountains. With a height of about 190 feet, this would basically be the equivalent of a 20 storey building.

I don't think you would save any notable amount of fuel. Granted, taking off from a down sloping runway would perhaps take a few seconds less compared to a flat runway, but the engines would still be running at near full power for the entire duration of the flight. The take off roll is only responsible for a fraction of the total fuel consumption, making any difference here hardly noticable.

Planes land and take off into the wind. If you wanted to build a sloping runway, you would have to build another, parallel, runway sloping the other way, otherwise the single runway would be sloping in the wrong direction whenever the wind came from a certain direction.

On landing, planes don't really use much engine power to stop.  Its mostly braking action.  (Sure, thrust reversers are available, to reduce stopping distance, but the major deceleration comes from brakes)

@J.Hougaard Are you sure about planes always taking off into the wind? LAX's runways all go east-west; I've seen countless planes approach the airport heading westward, and have never seen one departing the other way.

I take that back. My wife used to live under the flight path of LAX and she HAS heard planes going both directions!

@ShawnV.Wilson Yes, I am quite sure .. that's the kind of thing they teach you at ATC school :)

@J.Hougaard I believe you, but Carlo's answer refers to "one-way in / one-way-out runways".  Do they switch the "one-way" direction depending on the wind?

@ShawnV.Wilson That depends entirely on the local procedures. Obviously, if there is a mountain at one end of the runway, that is not really an option

Yes, it does work, but cannot really be called practical.
The first reason is wind: It helps to take off and land with a headwind. If the runway slope is on one end, this has to be the end where the take-off run starts and where the landing run ends. Since both are in the same direction, this scheme now needs twice the runway length, the first half for landings and the second half for take-offs.
The next reason is the magnitude of what can be saved. The energy from the height change $∆h$ can be translated into a speed gain $∆v$ by this formula: $$∆v = \sqrt{2\cdot g\cdot ∆h}$$
Let the ten stories be a height of 40 meters, and your speed gain is only 28 m/s. This was significant in the age of propeller aircraft, but jets need much higher speeds to get airborne - 150 knots or more. In sane units this is 77 m/s, and since the energy is proportional to the speed squared, the 40 m slope will save only 13% of the energy needed for take-off. It is simply not worth it.
Now look at the operational consequences: The landing has to be performed such that the aircraft has slowed down to 28 m/s when it reaches the foot of the slope. If it is still too fast, it will overshoot and roll down the other side, and if it is too slow, it needs to run up the engines in order to climb up the slope, expending the energy it hopes to save at take-off.
No, practical is not what I would call this.

@ymb1 I faintly remember reading about this. The whole terminal building was on a hill, and all runways would end sloping up to it. Would save some taxi fuel, too.

That's right; I left out that detail about the building. The landing runway came to the "south" side of the building, and the takeoff runway led to the "north". So you really could have runways in both directions.

Personally, I didn't like the idea of a building directly in the flight path, nor that the runways would be ramps that a plane could fall off. That's why (in my Question) I put them on the sides of a hill instead.

@ShawnV.Wilson: You are free to modify my description, and I thought of the terminal being offset from the runways. Placed on a gently sloping hill, there are no ramps, and I changed all occurrences of "ramp" in my answer to "slope" because it describes better what I thought of.

How about all the fuel needed for driving pax and cargo up that hill? Probably a lot more than what's being saved by the airlines.

It's never been planned as such due to the usefulness of flying approaches into the opposite end of the runway when winds favor so.
That being said there are a number of inclined one-way in / one-way-out runways in the world, mainly in remote and mountainous areas.  The most notorious of which is Courchevel airport in the French alps.  Like Gustaf III on St Barts, Courchevel requires special training and a logbook endorsement to fly approaches into.

Why do they choose to have to have planes take off and land going uphill? It's not to save landing fuel, is it?

No the airplanes land on the uphill and take off on the downhill.

That's the opposite of what I thought you meant by "one-way in / one-way-out"

Not practical for econoomic advantage, but there is an advantage if you are building airstips in the mountains - the strip doesn't have to be so long! many one-way strips in New Guinea with slopes up to 17%. As in the alps, commercial operations restricted to pilots with five trips under supervision, preferably under different weather conditions. An exemption, which I had, was supposed to require 1000 hrs in PNG and endorsement into 50 strips. I didn't get to that many .  Wind direction irrelevant, accept what it is and compensate. On the steeper strips, full power at touch-down or you won't get to the top.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What is the difference between DINS and the regular FAA NOTAM website?
Plain and simple, what the heck is the difference between the Defense Internet NOTAM Service (DINS) and the FAA NOTAM website? They are both located behind a faa.gov url.
DINS Example: https://www.notams.faa.gov/dinsQueryWeb/
"Regular" NOTAMS: https://pilotweb.nas.faa.gov/PilotWeb/
Thanks!

On the DINS site, it has this in red at the top of the page: ~ For DoD Flight Only ~

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the difference between en-route delays and airport delays?
ANSP performance is gathered an published online on https://ansperformance.eu. They monitor complexity, delays, efficiency and other operational data.
I also noticed they have two ways of describing delays with a formal definition:

En-route delays

The en-route ATFM delay provides an indication of ATFM delays on the ground due to constraints en-route.

Airport delays:

The Airport Arrival ATFM Delay provides an indication of ATFM delays on the ground due to constraints at airports.

My question is: What does this exactly mean? Are these different ways of describing the same delays? Or do they describe two different sources of delays.
One reason of my doubt is that both categories use the same classification, from A (accident/incident) - W (weather). Could someone shine some light on this?

If an aircraft has to divert around a major storm, that can delay the arrival time. The weather may be clear at the destination, but between "here and there" is a major storm.

The delays you mentioned are imposed by Eurocontrol to manage traffic flow all over Europe:

Even when an aircraft is ready to depart, it may still be delayed by ‘flow management’ (ATFM). You may well hear that your flight is ‘waiting for its Air Traffic Control slot’ and this is explained in another article. In short, if there is a problem that will delay an aircraft either en-route or on the approach to the destination airport, then it’s safer and cheaper to hold the aircraft on the ground, not burning fuel, and give it a delayed take-off time.
EUROCONTROL as Network Manager looks at all the flight plans and identifies where and when there are too many flights/too little capacity. This might be as a result of a peak in demand (aviation has rush hours too!), weather problems, limited capacity at the destination airport or many other possibilities. In total, flow management delays account for about 16% of total delays (in 2017), mostly relating to issues at the destination airport.

(Eurocontrol)

ATFM departure slots are allocated centrally by the European Network Manager upon the request of the local Flow Management Position (FMP), when an imbalance between demand and capacity is foreseen at airports and/or en-route. The purpose of such regulation is to hold aircraft on the ground.

(Eurocontrol ATFM Slot Adherence)
To the pilots, this comes as a so called CTOT (Calculated Take-Off Time):

A time calculated and issued by the Central Flow Management unit, as a result of tactical slot allocation, at which a flight is expected to become airborne.
An Air Traffic Flow & Capacity Management (ATFCM) departure slot, forming part of an Air Traffic Control (ATC) clearance, which is issued to a flight affected by Network Management regulations. It is defined by a time and tolerance (-5 to +10 minutes) during which period the flight is expected to take-off.

(Eurocontrol ATM Lexicon)
For each of these delays, there is an associated Reference Location where the reason for the delay is located. This defines, which delay category applies:

En-route delay:

Delay caused by regulation based on traffic volume which has a reference location classified as AS (Airspace) or SP (Special Point).
ATFM delay caused by regulations applied by the CFMU at the request of the FMP to protect en-route ATC sectors from overload.

(Eurocontrol ATM Lexicon)
Airport Delay:

Delay caused by regulation based on traffic volume which has a reference location classified as AZ (Aerodrome Zone) or AD (Aerodrome)

(Eurocontrol ATM Lexicon)

Examples for delaying a departure could be:

enroute weather does not allow to fly the filed route at the moment (En-route delay)
weather at destination is not expected to be good enough to land at the estimated arrival time (Airport delay)
there will be too many aircraft in one air traffic control sector (En-route delay)
there will be too many aircraft arriving at the destination airport at the same time (Airport delay)

Wow thanks. That is very clear, and quick. It is thus just a matter of source/location. So if I would like to investigate how ATC (e.g. staffing, performance) affects delay, I would need to look at both en-route and airport, because ATC can impact either, right?

@RobinKramer In principle yes, but I don't think ATC staffing is the limiting factor for an airport. That is usually coming from available runways or weather. En-route however, the number of aircraft one controller is allowed to handle at any time is limited: "The capacity of the current ATC system is primarily limited by the maximum number of aircraft that a controller can handle in a sector." (https://www.eurocontrol.int/archive_download/all/node/9730)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is the material that is used for turbine blades that has the highest melting point?
What is the material that is used for turbine blades that has the highest melting point ?

HPT blades are made (today) of noncrystalline nickel-based superalloys. Related: What material is used to make the hot sections of jet engines? They are cooled by air and may bear gas hotter than their melting point.

Are you asking for materials in general (including CMCs) or are you more interested in metals and their alloys?

Heavily related: Why are aircraft turbine blades made of nickel alloys?.

Gases temperature on the turbine blades can reach 1700-1800K. There is no material which can withstand such a temperature in conjunction with enormous centrifugal forces.
That's why blades are not only made of nickel superalloys, coated with ceramics, but also are "film-cooled".
They have radial micro-passages where compressed air from high pressure compressor is fed. Most advanced designs also have micro-holes along that passages on the leading and trailing edge of the blade, which are the hottest points on the blade. This enables cooling blades down with air (air from HP compressor heats up to 400-500K, while blades work at 600-700K) and also creates a film of protective air over blade's surface, which prevents direct contact of blade and exhaust gases. That's because most of the heat transfer occurs by thermal conductivity at direct contact, and much less - by heat radiation.


text:
What is the usual practice in interacting with ATC at Top-of-Descent?
Presuming an ordinary long-haul flight on which the plane's navigation system has selected a point as Top-of-Descent, and there are no particular issues (such as weather), when would ATC typically be contacted?  That is, would the pilots seek to obtain ATC clearance for descent in advance of reaching TOD so that they could start down at exactly that point?  Would the pilots cross TOD and then call ATC to ask for clearance?  Would there be a point when the pilots get early clearance to descend once the TOD point is reached but wait awhile to hit the TOD point?

When flying VFR you don’t have to tell ATC that you are changing altitude. But I always do and I tell them that I have the latest ATIS for my destination. It doesn’t hurt to let them know what you are doing and eventually they are going to ask you if you have the ATIS anyway.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Where can I find detailed information about ground lighting systems exist in aviation?
While I'm learning jeppesen charts, I find a list of symbols related to lighting ground facilities, there are a lot of weird abbreviation such as:
Standard ALSF - 1/2, MALSR, SSALR, MALS, SALS, RLLS etc.?
I don't want someone explain all of these to me(because there are a lot of them), but give me source, where I can find them all with depiction and explanation.
Which of these are widely used, and why?

You'll find a description in FAA JO 6850.2 Visual Guidance Lighting Systems, and Code 7700 has created a summary.

I know you said you didn't want to just be told what they meant, however in completeness of answering the question here are some abbreviations:

MALSR: Medium-intensity Approach Lighting System with Runway Alignment Indicator Lights
MALSF: Medium-intensity Approach Lighting System with Sequenced Flashing lights
SALS: Short Approach Lighting System
SSALS: Simplified Short Approach Lighting System
SSALR: Simplified Short Approach Lighting System with Runway Alignment Indicator Lights
SSALF: Simplified Short Approach Lighting System with Sequenced Flashing Lights
ODALS: Omnidirectional Approach Lighting System
ALSF-1: Approach Lighting System with Sequenced Flashing Lights configuration 1
ALSF-2: Approach Lighting System with Sequenced Flashing Lights configuration 2
CALVERT I/ICAO-1 HIALS: ICAO-compliant configuration 1 High Intensity Approach Lighting System
CALVERT II/ICAO-2 HIALS: ICAO-compliant configuration 2 High Intensity Approach Lighting System
LDIN: Lead-in lighting
REIL: Runway End Identification Lights
RAIL: Runway Alignment Indicator Lights

I found them here on wikipedia which has a nice entry on the subject

Check out the "Aeronautical Information Manual" available on the FAA website. Most of this are covered there with text and even system diagrams.


text:
How to deal with the noisy cockpit?
From the pilot perspective I want to know how to deal with the noise in the cockpit on the turboprop aircraft while long-run flights.
The reason is old type aircraft with bad headphones which doesn't protect from noise than they should be.

Are you not allowed to user your own headset?

The problem is that headset is very old, reducing noise approximately to 70%.

The most common way to deal with noise in the cockpit is to get yourself a noise canceling headset. There are a bunch of models out there but the most common seem to be the Bose and the Lightspeed. I personally have the lightspeed and will attest to at least a 30%-50% perceived noise reduction over my old passive headset.

I have the problem with connector which doesn't allow to install new headset. AN-26 aircraft

If the most common way of dealing with noise doesn't work for you, that's something you should definitely state prominently in the question.

A decent electrical engineer should be able to fabricate a conversion plug to allow a modern ANR headset to work in that aircraft. Well worth the money, imho, in order to preserve your hearing.


text:
What is the difference between microburst and windshear?
When I read about these weather phenomena, I find that both of them depict the same thing, and have the same actions to avoid. So why are they differentiated?

http://www.myairlineflight.com/windshear.html
https://en.wikipedia.org/wiki/Microburst

Because the microburst shaft of air hits the ground and spreads out in all directions, if you are descending toward it close to the ground you will at some point hit the outflowing air and experience an "increasing performance" shear (airspeed rise) as you descend into the outflow.  A sudden increase in airspeed while descending in the vicinity of a thunderstorm is a huge warning sign to get ready for fun as you fly into the shaft itself.  This is one of the things that triggers windshear warning systems on airliners.

A microburst is generally an intense down draft while wind shear can occur in any direction. It might be safe to say that a microburst is windshear but not all windshear is a microburst. From your linked pages

A microburst is intense small-scale downdraft produced by a thunderstorm or rain shower.

and

Windshear is any rapid change in wind direction and/or speed.

... and/or speed with altitude.


text:
What is the occasional crackling sound on an A330?
On a recent long-haul flight on an A330, at various points during the flight a strange crackling-type noise could be heard through the aircraft. It first happened whilst taxiing for takeoff and then subsequently every two hours or so.
If I had to guess, I'd say it sounded like ice-crystals or something being blown down the air-conditioning ducts. I'm no aviation expert (by a long shot) but I've never head this sound before and wondered what it could be?!

I'd have the hear the sound myself to make a better judgment of this.

The flight crew have discovered that the energy lift from their protein power bar will only keep them awake for two hours. The crackling noise is them opening a new one. If you don't hear the sound after two hours, notify your cabin attendant -- the pilots are asleep.

I just came from MVD to MAD on an Iberia A330-200. MVD weather was hot. First time ever that I've heard the crackling noise. Firstly during pushback, then during take-off. I honestly thought that some gravel was moving through the ventilation. The noise was coming from the front to the back of the plane. I have to admit that I was rather worried and after the seatbelt lights were off I went to the flight attendant who told me it was normal. I even considered reporting to the captain when we landed. Never experienced that before, but I am glad to hear it's likely to be normal.

I have recently flown on a China Eastern A330-200 from Shanghai to Beijing and that noise was heard around 10.000 feet both on the way up and on the way down. It sounds awful! Everybody was rather worried, including myself who, despite being a frequent flyer, had never heard that noise. At first I thought it was some fatigue issue of the aircraft skin while "inflating" as internal pressure increased.
But a few days later I flew on a Cathay Pacific A330-300 from Hong Kong to Jakarta, and after hearing that noise again, I asked a flight attendant who told me it was likely ice crystals in the air conditioning tubes that form when the outside air is very humid. She said she heard it many times and it's considered normal on A330's.

Aha! That's sort of the line I was going down, so thanks :)

I'm going to assume that you're describing the normal sounds of various pieces of the cabin interior rubbing against each other, or expanding/contracting due to thermal changes.
An aircraft's basic structure is a skeletal frame covered with an outer skin. Typically aluminum, but fiberglass-like composites or more advanced materials like carbon-fiber reinforced plastic are also used. All the stuff on the inside, from the seats, overhead bins, cabin walls, galley, even lavatories and so on are simply bolted in place, attached to the floor and other suitable structural members. These objects will all flex slightly on their own when the aircraft is in motion, and you may be hearing creaks in the floorboards or other items. (This is perfectly safe!) Further, because the aircraft will flex, there is spacing between each of these components to allow slight movement. These spaces are stuffed with rubber or foam pieces. It's possible that you're hearing friction between the foam and the objects, but it's probably just all of the items jostling around.
The air conditioning system consists of flexible ducting (including hoses with plastic and metallic film) and these hoses may expand/contract when airflow is changed, or when the temperature changes. So if some crisp outside air was brought in to cool down the cabin, you may hear those ducts crinkle a bit during the transition.
But, like others have said, without hearing the actual sound, this is only a guess.


text:
Why does the elliptical wing have the lowest drag?
I read that the elliptical wing has the lowest induced drag. Why is that so? I welcome a mathematical and intuitive explanation.

Related (and maybe duplicate): How is the induced drag calculated for a wing with elliptical planform? with a specific link to the Prandtl lifting-line theory.

Induced drag is caused by the backward inclination of the lift vector. Lift is defined as the aerodynamic force perpendicular to the flow direction, and since lift is created by deflecting this flow downwards, the resulting force is slightly inclined backwards.
The formula for the deflection angle (called downwash angle) $\alpha_w$: $$\alpha_w = arctan \left(\frac{2\cdot c_L}{\pi\cdot AR}\right)$$
To arrive at the drag, you now need to multiply the local lift with this angle. For small $\alpha_w$s the arcus tangens can be neglected, and we get this familiar looking equation for the backwards-pointing component of the aerodynamic force: $$c_{Di} = \frac{c_L^2}{\pi\cdot AR} = \frac{\alpha_w^2\cdot\pi\cdot AR}{4}$$
Now consider a wing which will do this unevenly over the wingspan. Some parts create a lot of lift and, therefore, a lot of downward deflection. Others produce less lift and less deflection. Due to the quadratic dependency of induced drag with downwash angle, the sum of all parts will always be higher when there are variations in the local downwash angle over span. Only the wing with a constant downwash angle will have the minimum induced drag for a given lift.
This is not necessarily an elliptic wing. By selecting the right twist (incidence variation over wingspan), any planform can be made to produce a constant downwash angle, albeit at only one angle of attack. Only the elliptic wing will combine the constant downwash angle over span with a constant lift coefficient over span such that variations in angle of attack still result in a constant downwash angle.
This is strictly true only in inviscid flow and when elastic deformation and the weight of the wing (which contributes to the need for lift and hence induced drag) are not considered. Also, an elliptic wing will produce nasty stall characteristics. Adding handling, friction and structural weight will shift the optimum to wings which produce more lift near the center. The exact optimum also will depend on the absolute size of the wing because the structural mass fraction goes up with size due to scaling laws.
Nomenclature:
    $AR \:\:$  aspect ratio of the wing
    $c_{Di} \:\:$  Induced drag coefficient
    $c_L \:\:\:$ lift coefficient
    $\pi \:\:\:\;$  3.14159$\dots$

+1: Thank you alot for your answer. Your statement "Due to the quadratic dependency of induced drag with downwash angle, the sum of all parts will always be higher when there are variations in the local downwash angle over span" was not so obvious to me at first glance. But now I have proved it and hence accepted it.


text:
What is the icing probability required for an AIRMET?
For an icing AIRMET to be issued, moderate icing must be forecast over an area of at least 3,000 square miles. What probability of moderate icing is required before that AIRMET will be issued?

For any AIRMET, the event must have a probability of 50% at least (that is more than playing playing heads or tails).
Related US NOAA definition for AIRMET Z:

AIRMETs are issued every 6 hours, where there is a 50% probability
  that any of the following en-route weather phenomena will cover an
  area of 3000 square miles or greater.

Phenomena for AIRMET Z are:

Icing: Areas of moderate airframe icing, other than convectively
  induced, including the areal extent. Freezing Level information is included after AIRMETs for moderate icing or statements indicating that no significant icing is expected.

Freezing level is defined as:

The lowest freezing level above the ground or at the SFC as
  appropriate. Freezing levels are delineated using high altitude VOR
  locations describing the location of the lowest freezing level above
  the ground or SFC as appropriate.
Freezing levels above the ground are delineated at 4000 feet intervals
  above mean sea level (AMSL). Multiple freezing levels above the ground
  are delineated using "BOUNDED BY" followed by high altitude VOR
  locations. The range of freezing levels across the forecast area is
  also included.

G-AIRMETs, the graphical version, are issued every 3 hours (but still describe the next 6 hours).

Thanks for the quick answer. This one stumped a number of people, including in follow-up to my asking this during a webinar on the subject. I'll take a brief wet noodle lashing for inadequate searching before asking the question here: I didn't phrase my searches broadly enough.

@mins is a much respected Hanshi master in the ancient art of Google-Fu. Do not despair. Few can match his skills.

Btw, your GPS knows where you are. The NSA knows what you're doing there. Google knows what you're thinking while you're doing it


text:
Why does GR mean hail in a METAR?
Wouldn't you think they'd choose HL instead?
That might confuse pilots who frequently see SS (Sandstorms),
 DS (Dust Storms) and VA (Volcanic Ash).

Related: Why is mist abbreviated BR on METARs and TAFs?

"Granite Rain".  No, not really, but it's a reasonable mnemonic.

And smoke is FU—from Fumée

The French word for hail is grêle which is abbreviated GR.  METAR is an international standard so you will see some non-English words included in the METAR.

Hail in spanish is Granizo, so probably from latin :D


text:
What does the Airspeed indicator show when your pitot tube is blocked?
So the question in the topic.
I want to clarify whether the speed indications is maintained or go to 0.

Note that you may have several pitot tubes, and thus several readings (which are expected to be the same if no pitot tube is  blocked) (one for the captain, another for the F/O, ...)

Depends on how much blockage. If fully blocked, as in air tight, the pitot side becomes like the sealed aneroid side of an altimeter and it only reacts to changes in static pressure, indicating the difference between ambient static and the pressure trapped in the pitot side.
If the blockage is not air tight, like you usually get with bugs, somewhere between zero and actual, in proportion to the blockage.  If the blockage is something that can move around, like a slug of water in the pitot line, you get wild and crazy fluctuations.

If the pitot tube gets blocked in flight, such as by icing, the airspeed indication is maintained at first.
Note that the indication is still subject to changes in static pressure as long as the static port is not blocked as well, effectively turning the airspeed indicator into an altimeter.
If only the static port is blocked, the airspeed indicator shows a weird combination of airspeed and altitude.

If the Pitot is ice over or plugged by a bug in flight, the ASI will indicate the speed at the time it was block as long as the pressure remains constant in the bourdon tube. The ASI will change with any altitude change, acting as a altimeter. If the is a climb the ASI will show an increase in speed, if descending there will be a decrees in airspeed.
 If the static ports a pugged or iced over the ASI will act a s altimeter in reverse.

In my case, the tube was blocked prior to takeoff (from insect debris). During flight, the airspeed indicator read a slightly negative number (analog gauge).

Sure it does... his aircraft had a blocked pitot tube, and this is what his ASI showed. It's less general than specific, but it directly answers the question as stated, imho.

It’s a better answer than the incorrect accepted answer.

Look at it this way:
The airspeed indicator is basically an instrument that provides the pilot with the difference between static and dynamic pressure, reported in a scale calibrated in a unit of speed (usually knots, which is nautical miles per hour; sometimes km/h or mi/h).
The static pressure is measured at the static port, and the dynamic pressure is measured through the pitot tube; in both cases, the one of each which is connected to the airspeed indicator. Very small aircraft will have one of each (unpressurized aircraft might not have a dedicated static port, but just be measuring that at or near the instrument), while large aircraft will have several of each.
When you're flying normally at a constant pressure altitude (say, holding a specific flight level), this means that the airspeed indicator reacts to changes in dynamic pressure, which in turn is basically how much air a given frontal area of the aircraft encounters per unit time. Go faster, dynamic pressure goes up, static pressure stays the same, difference increases so the airspeed indicator says your speed is increasing; go slower, dynamic pressure goes down, difference decreases, airspeed indicator says your speed is decreasing. Fine so far.
Now consider what happens if the pitot tube gets completely blocked, as in absolutely air tight on the side exposed to the ambient air. Whatever pressure there was before the blockage developed (for the sake of simplicity, let's say it got instantly and completely blocked by magic) will remain in the pitot system, but the static pressure measurement will still read ambient pressure correctly.
At that moment, for the airspeed indicator, nothing has changed; the static and dynamic pressure are both unchanged, and the aircraft is in fact still moving through the air at the same speed. So the airspeed indicator reports the same difference in pressure, ergo the same airspeed. It does not drop to zero.
Now consider what happens in such a situation when you climb or descend; in other words, when the static pressure changes.
When you descend, the static pressure increases (last I looked, air was thicker closer to the ground) but because it's blocked, the pressure in the pitot line remains unchanged. This causes the difference between the two to decrease, which as we established above is displayed by the airspeed indicator as a decrease in speed.
Correspondingly, when you climb with a blocked pitot tube, the static pressure decreases and the pitot pressure remains unchanged. Therefore the difference between the two increases, which the airspeed indicator reports as an increase in speed.
In other words, the airspeed indicator now acts as a crude altimeter! (In fact, reporting the difference between static pressure and a reference pressure is exactly what an altimeter is designed to do. It is, however, usually calibrated in a unit more useful for that purpose than knots...)
Both of these are the exact opposites of what would normally happen when you climb or descend at constant engine thrust.
If the pitot tube is only partially blocked, then some of the change in dynamic pressure will make it to the airspeed indicator, and therefore the difference will be less pronounced, but the error will still be in the same direction as if it was completely blocked at a given time.


text:
Why do aircraft need Mach number?
Why don't aircraft operates with IAS only?
Is there any other usage of Mach number except monitoring of shock waves formation?
How Mach number is calculated?

Mind you, not all aircraft need one, especially slower piston ones.
As you get closer to higher speeds, where compressiblity becomes significant, and you are approaching the speed of sound, the air starts to behave differently aerodynamicaly.
Also: remember how the air is a little bit accelerated over the wing because of the shape of the wing? Well, if you fly close to mach 1 and accelerate the air even further, a little bit of that air is now moving supersonically over the wing. And you now have a shockwave on your wing.
The plane might start pitching down (wing momentum changes), wing stalls (yeap, from too much speed), aerodynamic center moves etc, flight controls might stop working. Google ‘Mcrit’ and ‘mach tuck’ for more, but there are a lot of bad things generally happening in that region.
What it all boils down to is that the wing now has a built-in limitation by the engineers:do not fly faster than (say) mach 0.9 or you will be in trouble.
Now that we have a speed limit that depends on the speed of sound, we really need an instrument to show the pilots when that limit is approaching.

Very short explanation:
Formula to calculate Mach N.:   M = TAS / LSS
Where LSS = local speed of sound (remember that the local speed of sound is proportional to Temperature, lower T lower LSS. and it is a value expressed in Kelvin.)

To be precise: speed of sound is proportional to the square root of absolute temperature. Also slightly dependent on humidity. For practical purposes in aviation it is taken to be a function of altitude.

All the above is true, and just to add a little something:
above a certain altitude, all aircraft operate with Mach number and report to the controllers with that, that’s why you’ll hear quite often the control say: on conversion please maintain Mach .78 for example meaning when you switch from IAS to Mach keep that speed (here .78)


text:
Are there regulations for the field of view from cockpits?
Are there any design or ergonomic regulations with regard to the pilot's vertical and horizontal vision angles from the cockpit?
For example, pilot should see space 30 deg. above and 20 deg. below horizontal.

Not always.

The certification specification for aircraft are deliberately a bit vague on the topic. This allows manufacturers to come up with creative solutions that may have been prohibited if specific angles where required in the specification.
For large aircraft, EASA has requirements CS25.773 in the Certification Specification 25 (CS25) PDF which is included below this answer. The FAA requirements are similar in FAR 25.773.
To make sure that there is an agreed standard on what is acceptable to the certifying authorities, acceptable means of compliance have been published. Manufacturers may deviate from the AMC's if they have good arguments for it.
An example of the FAA AMC on this topic can be found in AC25.733-1 (PDF).
The FAA document refers to standard ARP 4101/2 which is the successor of SAE 580B (unfortunately you have to pay for these documents).
From this answer and the image there contained, you can see how the outside visual field of view of the cockpit of the A320 family compares to the 580B standard.

CS 25.773 Pilot
  compartment view (See AMC 25.773)  a) Non-precipitation conditions.
  For nonprecipitation conditions, the following apply:
(1) Each pilot
  compartment must be arranged to give the pilots a sufficiently
  extensive, clear, and undistorted view, to enable them to safely
  perform any manoeuvres within the operating limitations of the
  aeroplane, including taxiing, take-off, approach and landing.
(2)
  Each pilot compartment must be free of glare and reflection that could
  interfere with the normal duties of the minimum flight crew
  (established under CS 25.1523). This must be shown in day and night
  flight tests under non-precipitation conditions.
(b) Precipitation
  conditions. For precipitation conditions, the following apply:
(1)
  The aeroplane must have a means to maintain a clear portion of the
  windshield during precipitation conditions, sufficient for both pilots
  to have a sufficiently extensive view along the flight path in normal
  flight attitudes of the aeroplane. This means must be designed to
  function, without continuous attention on the part of the crew, in –
(i) Heavy rain at speeds up to 1·5 VSR1, with lift and drag devices
  retracted; and
(ii) The icing conditions specified in Appendix C and
  the following icing conditions specified in Appendix O, if
  certification for flight in icing conditions is sought (See AMC
  25.773(b)(1)(ii)):  (A) For aeroplanes certificated in accordance with CS 25.1420(a)(1), the icing conditions that the aeroplane is certified
  to safely exit following detection.
(B) For aeroplanes certificated
  in accordance with CS 25.1420(a)(2), the icing conditions that the
  aeroplane is certified to safely operate in and the icing conditions
  that the aeroplane is certified to safely exit following detection.
(C) For aeroplanes certificated in accordance with CS 25.1420(a)(3),
  all icing conditions.
(2) No single failure of the systems used to
  provide the view required by subparagraph (b)(1) of this paragraph
  must cause the loss of that view by both pilots in the specified
  precipitation conditions.
(3) The first pilot must have a window that:
(i) is openable under the conditions prescribed in subparagraph
  (b)(1) of this paragraph when the cabin is not pressurised;
(ii) provides the view specified in (b)(1); and
(iii) gives sufficient
  protection from the elements against impairment of the pilot's vision.
(4) The openable window specified in sub-paragraph (b)(3) of this
  paragraph need not be provided if it is shown that an area of the
  transparent surface will remain clear sufficient for at least one
  pilot to land the aeroplane safely in the event of -  (i) Any system
  failure or combination of failures, which is not, Extremely Improbable
  in accordance with CS 25.1309, under the precipitation conditions
  specified in sub-paragraph (b)(1) of this paragraph.  (ii) An
  encounter with severe hail, birds, or insects (See AMC 25.773(b)(4)).
(c) Internal windshield and window fogging. The aeroplane must have a
  means to prevent fogging to the internal portions of the windshield
  and window panels over an area which would provide the visibility
  specified in sub-paragraph (a) of this paragraph under all internal
  and external ambient conditions, including precipitation conditions,
  in which the aeroplane is intended to be operated (See AMC 25.773(c)).
(d) Fixed markers or other guides must be installed at each pilot
  station to enable the pilots to position themselves in their seats for
  an optimum combination of outside visibility and instrument scan. If
  lighted markers or guides are used they must comply with the
  requirements specified in CS 25.1381.

Many aircraft have eye position gauges in the cockpit that pilots can use to adjust their seat for optimal view. They either try to line up little colored balls or look through a small sight. Here are a few examples:
Airbus A320

Image Source
Cessna Citation

Image Source
ATR-72

Image Source
McDonnell-Douglas MD-80

Image Source

Yes, specific norms related to the cockpit is made by references of angles of elevation and depression so that no pilot feels that they are only running with dependency on various instruments rather than their own eyes. By considerations of the designers and testing pilots, The angles of vision of pilot from the cockpit are suitably chosen by calculations and testing . Also, most of the planes of same class have same angles like airbus a380 and Boeing 747 or an-32 and c130j super Hercules etc.

As mentioned in this answer: yes there are regulations for the vertical view from the cockpit. A complete horizontal and vertical view regulation has been proposed, but has been watered down to an Advisory Circular after protests from the industry.

Where does this drawing (T-tail airplane on approach) above come from? I have seen it cited before but without a reference.

@GregR.  I converted your answer (which, as you recognized, isn't properly an answer) into a comment on this post. That has the best chance of getting you an answer to this follow-up question, probably better than the new question that you posted. (I realize that you may not have the reputation yet to post the comment on your own -- I'm not suggesting that you did anything wrong here.)


text:
What was the first airplane fitted with a hydraulic system?
What was the first airplane fitted with hydraulic system?
Where was it used?
What are the fundamental differences between the first systems and those that are used now?

Hydraulic brake systems are a type of hydraulic system. Do you want to include or exclude those from your question?

Include of course

@wiaim I ask because hydraulic brakes are commonly separate systems from the "hydraulic systems" that are often discussed. The meaning is not obvious; I would have presumed that you meant to exclude braking systems.

What was the first airplane fitted with hydraulic system?/Where was it used?

If we exclude hydraulic disk brake systems.
The Lockheed Constellation most likely had the first hydraulic flight controls (introduced in '43)
Introduced in '27 the Fairchild FC-1/FC-2 had hydraulically operated landing gear and disk brakes.

What are the fundamental differences between the first systems and
  those that are used now?

Depending on how you look at it, little. Hydraulics are hydraulics. The systems have gotten more powerful but that may be related to the available power to drive the pumps. We arguably have gotten better at building them and they are perhaps more reliable but they are still more or less the same in operation.

The DC3 introduced in 1936 had multiple hydraulic systems including for brakes, gear retraction, flaps, and cowl flaps. I know the earlier DC2 also had hydraulics, at least for gear retraction.

The Fairchild FC-1/FC-2 had fixed, conventional gear. In what way were these hydraulically operated landing gear?

@JonathanWalters Im still trying to figure it out but multiple sources state that it had hydraulically operated landing gear.

It may have had "hydraulic" shock absorbers but it couldn't have hydraulically operated gear. They are hydraulic in the same sense the shocks/struts in your car are hydraulic...


text:
What is a transmit-only frequency?

I found the above in the Jeppesen Airport Chart Legend.
What would be an example of a transmit-only frequency? Is it related to Flight Service Stations? If so, how?
Where can I find an example for transmit-only frequency on a chart?

Are you talking about certain frequencies usually shared with navaids that are transmit on one frequency and receive on an other?

Maybe when the same ATC operator can use multiple frequencies, and cross-couple them (retransmit one to the other) or disconnect cross-coupling (to prevent an aircraft to RFI others). Seems to be used in Indianapolis terminal aera.

Would GUARD be an example of this?


text:
What's the difference between racetrack and holding pattern?
Is there any difference between them or they are only synonymous?

I was expecting a joke about a race ending after so many laps...

What's the difference between racetrack and holding pattern?
  Are they only synonymous?

When used in the context of holding, racetrack pattern and holding pattern refer most of the time to the same pattern, because a holding pattern is a usually designed as a racetrack. Holding terminology:

Holding (or flying a hold) is a maneuver designed to delay an aircraft already in flight.
A holding pattern is the pattern used to hold, regardless of the design of the pattern.
A racetrack holding pattern is a specific design of holding pattern. From Wikipedia:

A holding pattern for instrument flight rules (IFR) aircraft is
  usually a racetrack pattern based on a holding fix.

Holding patterns are defined relative to a holding fix and are usually timed because aircraft have to remain within a given protected space around the fix.
However a holding procedure which uses a racetrack pattern and a racetrack procedure which uses a racetrack pattern are two different things aimed at two different purposes.

Racetrack procedure
ICAO makes a specific use of the phrase "racetrack procedure" in Doc 8168 (PANS-OPS):

Racetrack procedure: A procedure designed to enable the aircraft to
  reduce altitude during the initial approach segment and/or establish
  the aircraft inbound when the entry into a reversal procedure is not
  practical.

and

A racetrack procedure consists of: a) a turn from the inbound track
  through 180° from overhead the facility or fix on to the outbound
  track, for 1, 2 or 3 minutes; followed by b) a 180° turn in the same
  direction to return to the inbound track.

For ICAO, a racetrack procedure is a turn procedure, or course reversal procedure, in the instrument approach procedure, comparable to other procedural turns, and has nothing to do with delaying the aircraft. The procedure turns come in different flavors:

Source.
Holding procedure
Holding procedure definition from ICAO Doc 8168 (PANS-OPS):

A predetermined maneuver which keeps an aircraft within a specified
  airspace while awaiting further clearance.

It can be used to delay the aircraft, but also to change its altitude using a holding pattern and stack. Note there is no reference to a specific pattern.

Source.
However, when holding, the pattern followed is often a racetrack pattern (more details about this in: How is a standard holding pattern flown?)

Depiction on instrument plates

Holding patterns are depicted using regular lines, they are optional
Procedure turns patterns are depicted using bold lines, meaning they are mandatory. Exceptions are noted "NoPT" with indication of the arriving course.

Adapted from AirNav.com.

Where I can find information about procedure turns?

@wiaim: You may start with FAA documentation, page 4-49 of Instrument Procedures Handbook (which has been referenced in @ JScarry 's answer). Then you may find numerous documents, e.g. by searching instrument approach "procedure turn"|"course reversal".

An important practical distinction is that a holding pattern includes the necessity of correcting for wind drift in order to continually circle back to cross one specific fix and remain in protected airspace. A racetrack pattern does not alwayd include that necessity, though other applications such as aerial refueling often do.

Holding patterns are used to delay aircraft when there are other aircraft ahead of them for landing at an airport. The holding pattern looks like a racetrack with either standard right turns or non-standard left turns.
FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge

Holding pattern. A racetrack pattern, involving two turns and two
  legs, used to keep an aircraft within a prescribed airspace with
  respect to a geographic fix. A standard pattern uses right turns;
  nonstandard patterns use left turns.

Holding patterns are also used on an approach to reverse course and possibly to lose altitude.
FAA-H-8083-16 Instrument Procedures Handbook

If the course reversal is depicted as a racetrack or a teardrop on the chart you must fly it as depicted. Otherwise you have a choice of how to fly it.
AIM 5-4-9 On U.S. Government charts, a barbed arrow indicates the maneuvering side of the outbound course on which the procedure turn is made. Headings are provided for course reversal using the 45 degree type procedure turn. However, the point at which the turn may be commenced and the type and rate of turn is left to the discretion of the pilot (limited by the charted remain within xx NM distance). Some of the options are the 45 degree procedure turn, the racetrack pattern, the teardrop procedure turn, or the 80 degree <--> 260 degree course reversal.
Holding patterns are also used when flying a missed approach. I don’t recall ever seeing a missed approach hold that is not depicted as a racetrack.

No, if a procedure turn is authorized, you must fly it in the manner depicted by the procedure turn barb on the plate.

@CarloFelicione I'll clarify my answer. If there is a procedure turn barb, you must fly it in the direction of the barb, but you have a choice of how to do the procedure turn. If it is a teardrop or racetrack, you have no choice as to  the type of turn. Entry method for racetrack is up to you, but there are suggested methods for each direction of approach.

One in the same.  Sometimes a traffic pattern around an airport can be called a racetrack or a circuit depending on where you are.


text:
What is the shortest regularly scheduled commercial flight?
Interesting to know is this flight profitable or not.
Why this route exist?
Maybe train or car will be cheaper?

Related: What is the rationale behind very short distance flights?


text:
How does the 3 pin ground power connector (AN2551) work?

I understand the premise of the 3rd (smaller) pin (top of the image) is to delay switching power until the two main pins have made positive contact with the aircraft socket on plug insertion; and similarly, to disconnect power before the two main pins lose contact on plug removal.
This setup is desirable as it;

ensures correct polarity as the plug only fits into the socket one way
avoids arcing on insertion and removal which in turn;

removes a potential source of ignition (especially if refuelling)
prevents damage to the socket and pins

My understanding is that all pins on the plug are always live and that all control logic and switching is part of the aircraft's electrical system. ie: the switching occurs internal to the aircraft between the ground power socket and the main bus via a relay (powered by the external power source) that is ambiguously referred to as the external power relay (referring to its function, not its location).
I have several questions;

If the plug pins are always live, why does the third pin not present a potential arcing risk? (I assume the lower power has something to do with this but you can get a reasonable spark from a 9V battery).
How is the power on the 3rd pin limited?
Why not use the 3rd pin to switch the power at the source so that the positive pin is only ever live when connected?
eg: short the 3rd pin to the ground pin on the aircraft socket to activate an external relay (part of the power supply - not to be confused with the relay in the aircraft referred to as the external power relay) to switch the power on/off before it even reaches the aircraft. As a bonus this would also  (slightly) simplify the aircraft electrical system.
Is all the control logic always in the aircraft?
Is any aircraft power required? ie: will this work if the aircraft battery is completely dead?
Is the aircraft battery required to be physically connected (to complete the circuit for the relay)?

Sources that got me this far:

https://www.askmehelpdesk.com/aviation/3-pin-ground-power-unit-connector-aircraft-375477.html
http://aircraftplugs.com/download/i/mark_dl/u/4008094775/4627385351/purpose_of_third_positive_terminal.pdf
https://www.industrial-electronics.com/images/aircraft_8-9.jpg

Related: What is the process to disconnect ground electric power before pushback?

The key to how it works is best explained with the 3rd link in OP question: the short pin provides power for a relay switch, powered = closed. When pulling the plug out the short pin disconnects first, opening the switch which removes power from the plus (middle) terminal of the plug before it is disconnected from the counter-plug.

Well, the power does not neccessarily be limited on the 3rd pin - and neither is there any substantial arcing, as it is plugged in and out without any substantial load (unless operated wrongly) - on plugging in, the external power switch should be still turned off, so nearly no current will flow (except to maybe a indicator light - but that has a low inductance, and thus little induced voltage).
Having the control inside the aircraft makes for a simple construction with very limited room for operational error - switching the power outside the aircraft might be a feasable option, but keep it simple...
Yes, this works without any power on the plane itself, though subsequent electrical systems might need the battery to be present (and with some residual charge) to operate.


text:
Are airliners designed to land safely with no undercarriage deployed?
Complete undercarriage failure ("belly landing") is very rare in large passenger aircraft, but not unprecedented.

(source)
I would imagine it would be fairly easy to predict which part of the fuselage and engines would be in contact with the runway in such a situation. Are airliner fuselages and / or engines designed for this eventuality, so that the event is more likely to be survivable? Or is it considered too rare for structural modifications to be cost-effective?

Note: planes are designed to safely "land on water", which requires a similar design. And IIRC modern aircraft must have detachable engines (in case of heavy contact to the ground). And aircraft are designed for evacuation of passengers. (for sure not only a soft landing with fire on lavatory)

Yes, airliners are designed to land safely with no undercarriage deployed, if "safely" is to be considered synonymous to:

...give each occupant every reasonable chance of escaping serious injury in a minor crash landing...

Part 25 - Airworthiness standards: Transport category airplanes Subpart C Emergy Landing Conditions deals with this. Specifically the § 25.561 General lays out the requirements for transport category aircraft design in the following manner:

(a) The airplane, although it may be damaged in emergency landing conditions on land or water, must be designed as prescribed in this section to protect each occupant under those conditions.
(b) The structure must be designed to give each occupant every reasonable chance of escaping serious injury in a minor crash landing when—
(1) Proper use is made of seats, belts, and all other safety design provisions;
(2) The wheels are retracted (where applicable); and
(3) The occupant experiences the following ultimate inertia forces acting separately relative to the surrounding structure:
(i) Upward, 3.0g
(ii) Forward, 9.0g
(iii) Sideward, 3.0g on the airframe; and 4.0g on the seats and their attachments.
(iv) Downward, 6.0g
(v) Rearward, 1.5g

(emphasis mine)
So regulations explicitly demand that the structures of the aircraft are designed such that they provide (reasonable) safety to occupants even if the plane landed with the landing gear up. Also worth noting is the use of words "minor crash", which means the gear up landing does not have to be fully controlled and smooth.


text:
Could the Lightning's overwing fuel tanks be safely jettisoned in flight?
The English Electric Lightning could be equipped with removable overwing external fuel tanks (I believe the only aircraft ever built with this feature).
 English Electric Lightning F6 source
Were they designed to be safely jettisoned in flight? Perhaps it was necessary to be inverted to do so.

The answer is yes they could be jettisoned, but with extreme caution because doing so with anything but empty tanks could jeopardise the airframe. There is an apocryphal story that attempting to jettison full tanks would result in them staying where they were and it was the wings that were jettisoned.
Each tank could carry 260 gallons (approx. 2000lbs) of fuel each.
 source
You'll notice the triangular warnings on both the support and the tank, indicating to ground crew that caution was needed when adding and removing the "overburgers".
According to the Lightning Aircrew Manual the procedure for fuel dumping was:

A guarded, two-position OFF/FUEL DUMP switch is on the left console of
the F Mk 6. Fuel is dumped through a pipe running from the bottom of
the centre compartment to the aft end of each overwing tank. The pipes
are sealed by a blanking cap held in position by a hinged lever
connected by two explosive bolts. When FUEL DUMP is selected, the
explosive bolts of both tanks fire to free the lever which hinges open
under the influence of a spring. Air pressure then dumps the fuel
overboard, the transfer in the tank to its centre compartment
following the normal sequence. From full, the tanks take approximately
two minutes to empty, and a further two minutes for residual fuel and
vapour to clear.

It later notes

Overwing tank fuel may be dumped at speeds between 220 knots and the
maximum carriage speed at heights up to 36,000 feet. Dumping is
normally to be performed in straight and level flight but turns and
not more than +2g may be made if necessary.

Dumping was a necessary pre-condition of jettisoning the fuel tanks. The manual doesn't provide any emergency scenarios in which such an action might be necessary, but does say:

Empty overwing tanks may be jettisoned between 200 and 250 knots and
up to 0.75M at heights up to 40,000 feet in straight and level flight.
Overwing tanks containing fuel are not to be jettisoned.

Here is XN725 during testing of the tanks, with large fins to ensure clean separation under various loads.
 source

According to Wikipedia:

The Lightning F.6 was originally nearly identical to the F.3A with the
exception that it could carry two 260-imperial-gallon (1,200 L) ferry
tanks on pylons over the wings. These tanks were jettisonable in an
emergency, and gave the F.6 a substantially improved deployment
capability.

For certain values of "safely". Nothing you do in a Lightning is ever really safe.


text:
Why does the Eurofighter Typhoon pitch up on brake release?
In the video here of an airshow take-off, it's noticeable that when the brakes are released there's a brief but pronounced fluctuation in pitch as the aircraft accelerates.
This is something you see in very fast accelerating cars / dragsters, but I believe that is caused by the torque through the rear wheels, which is obviously not the cause here.
The same thing can be seen on an F-16 but much less pronounced.
Certainly for the Typhoon, the nose-wheel oleo doesn't seem to be particularly compressed as it's held under braking. I'd guess that you want to ensure that there's no chance even the slightest bit of any upward vector in the engine thrust while you sit on the tarmac, otherwise your brakes might not work, but that doesn't really explain the wobbling.
 source

That is a high-performance take off. My Cessna does the same thing when I do short-fields.

This is something you see in very fast accelerating cars / dragsters, but I believe that is caused by the torque through the rear wheels, which is obviously not the cause here. You're conflating the source of the torque with the result of the torque on the vehicle. It is indeed the same reason for a vehicle pitching down when braking.

Does the downvoter care to explain?  I think it’s a great question!

I think this Q&A has been here before...

Interest only: Some relevance but peripheral. Under braking most vehicles "nose dive".  This is true of systems with suspension acting directly on the wheel suspension plate (eg most motorcycles)with tubular springs and "forks") . Vehlcles with trailing link suspensions also usually dip. But vehicles with leading link suspension usually rise on brake application - Sachs dirt bikes 'of my youth" being the 1st example that brought this to my attention. This is a most unnerving effect when first encountered but actually may be quite useful in stay-on-bike-limiting heavy braking situations.

Basically because it's pushed up by the suspension of the nose gear

When the brakes are on, they apply a backwards force that counters the engine thrust. This force is applied at ground level, and the engine thrust is higher. These two forces result in a moment that forces the nose down, compressing the nose gear slightly.
When the brakes are released, the moment disappears and the nose gear extends.
Another factor here is that the Eurofighter (and F-16) has its air-intake under the fuselage, which forces the designer to put the nose gear behind the intake, closer to the main wheels than other aircraft. That means the nose gear has a shorter lever-arm to resist pitching. It's set up with the same spring rates and damping because it's primary job is to react to bumps in the ground, so it's more sensitive to pitching.

Look carefully and you can see the nose strut is compressed down to only a few inches of chrome as the thrust applies a torque moment, or nose down pitching moment, about the fixed axis of the  main tire contact points with the runway.  On brake release, the axis about which the torque is being applied, the rubber to runway interface, is also no longer fixed, so the pitching moment compressing the strut drops drastically, not to zero, because there is still rolling friction, but way less.
With some or most of the nose down moment relived, the strut rebounds and over extends at bit, but there is still a nose down moment present (of the thrust acting about the rubber/runway contact point, but that is now only inhibited by rolling friction) that re-compresses the strut part way.
So the pop up and down again is from the strut releasing its stored energy as the pivot axis is un-fixed, overshooting, and recompressing and stabilizing in reaction to the new reduced down force of thrust acting about a "rolling friction axis" you might say.

Here's what's going on.

You have two major compressible elements on the landing gear: the "oleos".  Movement is not possible anywhere else.  You can imagine a line straight through them.   The center of rotation will be somewhere between the oleos along that line, based on the ratio of spring resistance on each one.
You see the line of thrust is not down that line.
Since the line of thrust is off the center of rotation, the ship must tip forward (nose down).
If it were (i.e. the engine was podded and hung way down), then indeed thrust would be inline with the strut line.  The struts would endure lots of side load, but they do that already; they would have no desire to compress or expand.
The brakes, wheels and tires aren't even players here, because they are immobile, pinned, shocked, tugged, rusted solid, or held.  This is drawn as if those are missing and the airplane is plinthed below the oleo's (e.g. the VC-137 at the Reagan museum).  It would still do this.
If the ship is suddenly un-plinthed and able to slide or roll free, then everything changes. The thrust contributing to nose-down instead creates forward force. The oleos, now not resisting thrust (or barely), spring back to a more neutral position.

So you are saying that releasing the brakes has no effect on the aircraft's pitch attitude?  I think you may be mistaken.  I may be misunderstanding you as I have never heard the word "plinthed" before.

@quietflyer A plinth is a permanent mounting for display; aircraft are regularly mounted on them, sometimes via gear, sometimes via direct body attach. Note that I have drawn the aircraft as if plinthed or attached to a tug. Yes, a held or rusted-solid brake would be equivalent.  As such, the only possible movement is in the oleo's.   The ability to roll or slide in low-friction would allow movement elsewhere, and would change everything.

I think both answers here are correct, but I want to attempt a simple explanation.
Take the image below. Think of the stationary jet as a lever, with the rear wheels as the fulcrum. When the engines are throttled up, it causes the lever (the body of the jet) to rotate about the fulcrum. This pivoting is what compresses the front gear.
This is almost identical to why a car's front will pitch down when you brake.

Eurofighter source
Lever source

Your force diagram has a vertical force.  The jet engine is pointed perpendicular to the weight and the vertical force comes from the height difference, like Robin explained.  The analogy to cars is potentially misleading unless you already understand exactly what's happening here; yes it works when cars are slowing down from speed (because their center of mass isn't at ground level).  But not when cars are revving their engine with brakes applied.  (Automatic transmission only).


text:
Do GA IFR flights out of airports w/ SIDs typically use them? What about STARs?
Besides the IAPs (which as an IFR flight you're guaranteed to use unless cleared for the visual), there are the STARs and SIDs, and using them in slow GA aircraft doesn't fully make sense to me.
For example, out of RIC there is the Colin Six Departure:

I would imagine that as a 150 KT puttering IFR GA aircraft you would be literally holding up any heavies behind you on a busy day, heading for that COLIN waypoint, that are trying to push the < FL010 250 KT speed limit.
Same for the STARs: your gentle 185 KT descent through 3,000 is a cork for the 747 at 240 KT on the same path but 10 NM back and 1,000 feet higher, getting lower and closer.

Does this simply come down to ATC's ability separate aircraft, and
Are these standard instrument approaches and departures always/often/at all used by small & slow IFR GA aircraft?

Please clarify what you mean by General Aviation (GA). The definition of GA is variously given as all civil aviation excluding: all scheduled airline flights; all airline or charter flights; or all §121 and §135 flights.

Sorry, I may have been using GA wrong for a long time then. I use it to refer to basically any non-commercial flying, cargo or pax.

I see; that is a common definition. But even that definition also includes a lot of private business aviation.

True. I'll stop using it as a catch-all then and find more specific phraseology. Thanks!

I cannot speak for all of General Aviation (GA), but throughout the IFR portion of my flying career I have generally used SIDs wherever available.
NOTE: This choice of using SIDs was not always mine, as ATC will often include a SID in a clearance unless requested otherwise. A pilot has two opportunities to avoid SID's if so desired: firstly, when filing the IFR flight plan the statement "NO SIDS" can be included as a request; secondly, when receiving the IFR clearance, the SID can be declined.
I typically like using most SIDs, and I understand that ATC also likes giving them as part of a clearance. Many SIDs are essentially just radar vectors in normal usage with further instructions in case of lost comms.
My experience with SIDs has included numerous aircraft with typical climbout speeds ranging from 90–280 KIAS. As far as I have been able to tell, my aircraft's speed has never been a problem that ATC couldn't handle.
I have less experience with STARs. In flight training I used them occasionally, and ATC seemed to handle my slow speed without issue. At my current operation—which you may or may not consider to be GA—ATC rarely gives of STARs, but usually just gives us clearances direct or vectors. I personally like filing STARs since it helps me know what to expect.

A note on General Aviation (GA): multiple uses exist for the term, "general aviation" (see my query below your question). All of the definitions I am aware of would include the flights in smaller single and multi-engine aircraft that I have flown during flight training. Some of the definitions I am aware of would include some of the turboprop flying I have done (non-charter §135), some would not. But I think that all of the definitions I am familiar with would include private business aviation under §91—which includes some of the fastest civilian jets out there.

Great response thanks! As a follow-up question or two:

Do you always perform the entire SID? For example, in the COLIN SID I posted above, performing the entire thing would take me a hell of a distance in a very specific direction which may not be at all optimal for my destination, and there isn't a "better" SID available. Same question concerning the altitude (in the example 5000 ft).

What percentage of the time do you have ATC vector you completely differently from your filed plan, e.g. different SID or not a SID at all? And finally

Do you use SIDs and STARs at all while VFR?

1 No, I often don't perform the entire SID. I sometimes get vectors off the SID or more frequently get a "direct to" my destination or a waypoint further down the flightplan. Departure control is usually very good at getting me heading in the right directly out of there airspace; they want me out of their hair.

To specifically address the example of the COLIN6, KRIC does have several other SIDs, but they all take you either east, northeast, or west. If you wanted to go south, ATC would probably vector or otherwise get you heading in the right direction sooner.

2 In my experience ATC rarely, if ever, sends us very far out of our way. I have never gotten a SID that would send me in the wrong direction very far. One example is a SID that sends me in the wrong direction about 15 miles, and if I have to fly it far enough I will. But in that case, once I have climbed high enough ATC usually gives me direct my destination in the opposite direction.

@Answoquest 3 I have never a STAR under VFR, but I have used or consulted SIDs for guidance when making a VFR climbout in mountainous terrain.

Thanks for all the detailed answers! Last couple: When you file your flight plan, since you don't know for sure which runway ATC will clear you for or the SID they will offer you, do you still enter one in the plan? e.g. KRIC READE4.MOL for the READE 4 SID w/ the Montibello transition. What if you want to use the SID but without one of the transitions?

@Answoquest Filing for, or receiving clearance for, a SID does not require that you know your departure runway. In the case of the READE4.MOL, I could file just that, and then regardless of which runway is used, the SID will direct me to CARML and thence onward. If I didn't want to use one of the other transitions, I would just file READE4.READE, and plan to transition to enroute from READE.

Perfect, thanks again!

I fly a light single-engine airplane (Mooney), and often get assigned a STAR. When the weather is IMC, I typically fly the full STAR until ATC starts to vector me for the approach. If VMC, they often give me a shortcut (direct to a VOR near the destination) and tell me to expect a visual approach.

Some SIDs/STARs have a restriction to only turbojet or turboprop aircraft and say piston aircraft (what I suspect you meant by "GA") should use another SID/STAR, or they may have different altitude or speed restrictions (or "expect"s) for different types.
Some, like the one shown above, don't say anything about aircraft performance; this probably means that IFR traffic levels there are low enough that ATC will just vector slower traffic off the SID/STAR in the rare cases when they do actually cause a problem for faster traffic. But they still want everyone to be cleared for the SID/STAR because then both sides know what the other expects in the event of lost comms.


text:
What is this instrument for on the 747 flight engineer's panel?
Above the flight engineer's panel in a 747-100 there is an instrument that looks like a curved spirit level.
What was it used for?

source

Looks like an inclinometer as using in sailing, though with less heel range.

It's for measuring the airplane's inclination on the ground for the longitudinal axis. There will be another one on a lateral bulkhead for measuring lateral inclination.
When doing fuel quantity calculations from the manual dipsticks, you have to take into account the effects of the plane's tilt on the fuel depth measurements, so you will read the values on the inclinometers and consult some kind of documentation that applies corrections for off-level conditions.
There are also other situations where they come in handy, like when you want to park in a level spot, or to keep the plane level during jacking.
Just about all Transport Category airplanes have inclinometers like that somewhere in the cockpit.

Would such an instrument offer more precision than other banking indications, or is the purpose of having the instruments their ability to operate even when a plane is powered down?  Would the instruments be used to conform calibration of inertial instruments?

In flight it will show deck angle if you're flying steady state, but only steady state and would be totally useless for showing pitch attitude. The lateral one will show slip/skid (not roll angle) like a skid ball on the instrument panel, but can't show bank angle.  As far as using them to try the fly the plane, they are fairly useless. They're only utility is showing the long and lat axis levelness on the ground.  They aren't a requirement for initialization of Inertial Reference Systems.

I wasn't thinking that they would be useful in flight, but that they might be useful when the plane is stationary on the ground as part of a pre-flight check (if the artificial horizon shows a half-degree bank to the left, the inclinometer should do likewise).  Otherwise, would the advantage over the artificial horizon be the fact that it's always ready for use, or would it have other advantages as well?

The gyro instruments (the actual gyros are remote in black boxes under the floor somewhere), erect themselves and don't need any help from the pilot. As far as cross checking,  modern airliner has 3 separate attitude gyros, and if there is a disagreement between them, the warning systems tell you about the miscompare.  When I was flying RJs we never consulted those clinos, because the only time the flight crew would ever use them is measuring fuel quantity with the magna sticks b/c the fuel quantity system is US and I never had that problem.


text:
What is this military aircraft from the James Bond film Octopussy?
The James Bond 1983 adventure Octopussy opens with Bond flying through a hangar in a Bede BD-5 and generally causing carnage. One of the planes I cannot identify.

It is possible that it is a smorgasbord of aircraft bits, of course.

There is such thing as the Internet Movie Plane Database which has a page for Octopussy
It includes this picture:

which it lists as:

Modified nose section of a Dutch built Hawker Hunter, serial N-202. Also seen in The Man with the Golden Gun.

A quick google will show that those are not the inlets of a Hawker Hunter -- unless substantial modifications were done.
The inlets look quite small - and the splitter plate seems very close to the side of body.  Maybe it is a bit of Hollywood magic.

I was unaware of the IMPDB, thank you.   There's a IMcarDB and a IMfirearmsDB in addition to the original IMDB; I wonder what other databases of movie "things" there are....

@RobMcDonald I agree it looks nothing like the inlets of a hunter. Just repeating a piece of evidence I found. I suspect its just the props department making it look more aeroplane-y

The inlets bear a passing resemblance to a Jaguar, which was a bit newer at the time. Wouldn't be a surprise if the props guys "updated" it with plywood and filler, as long as it was never going to fly again.

Sure, it would be a commonplace that, in passing, the director or art director might randomly comment to a grip or someone "throw something on that hole thing there so it looks cooler".  (note that movie "magic", ie cgi etc, was not part of the universe then.)

No good engineer would fit a straight splitter plate to a rounded fuselage side. Those clearly were added by a gifted amateur.

@PeterKämpf: the ones of the F-4 were actually straight... but it were in the '50s

The F-4 splitter plates were straight because they were hinged in the middle to allow them to be used to modify the inlet flow at speeds above about M1.6 to ensure subsonic flow at the compression face.

I think the short answer is it's not a real aircraft. It looks like a prop. It bears the closest resemblance to me to the McDonnell Douglass F-4 Phantom II, but shares a few commonalities with the Hunter also (might be a prop built around the remains of N-202 as suggested above?).
One indicator it might be a F-4 is the inverted-teardrop-shaped flat front cockpit glass which was common on US fighter aircraft of the era such as the F-4, A-6 Intruder (in a distinctive side-by-side pair) and F-14 Tomcat which allowed a clear FoV for the HUD, but this glass shape was also fitted on a lot of Hunters for the same purpose, and the cockpit itself looks very Hunter-shaped and is only a single-seater.
Another is the large side intakes on the fuselage with the 'barge board' up against the cockpit (I don't know the actual term). The scale and proportion of these are not particularly accurate to the F-4, but the shape is very distinctive.
The nose also has the wrong profile and taper for a Hunter, but it might have been altered to look more 'modern'. It looks a lot more like a F-4 Phantom nose with the black nose cone being a common feature on early F-4s.
In the lower picture it also looks like it has a catapult bar on the nose gear (although the gear itself doesn't look carrier grade!)
Obvious 'not and F-4' things are lack of the tandem 2-seater cockpit, I can't see well in the screen-grab but it looks like the wing profiles are way off.
EDIT: Looking more deeply into it, it may also have drawn inspiration from the Avro Arrow and British TSR-2 prototypes.
Definitely a 'smorgasbord of aircraft bits' and a lot of imagination that's sort of an F-4 but not really!

Note the super-realistic glowing u-shaped tube in the radar dish!

That’s so the technician can see in the super-dark poorly lit hangar xD


text:
Is an aircraft capable of flying at 900 km/h at 8000 metres above ground?
There was a question in my maths textbook and I wondered if it was actually possible.
P.S. 8,000 metres is about 26,000 feet.

Yes, it's normal.  What is your doubt?

And there are aircraft that can go faster at much lower altitudes. 900 kmh is only about 560mph.

Is "above the ground" a trap? If you fly at 8 km above Everest you are already at an altitude of 17 km. I wonder what is the reason to have such question in a maths textbook. It can't be answered correctly without knowing a bit of aerodynamics, the difference between groundspeed and airspeed, the characteristics of the standard atmosphere, etc, and I don't see the mathematics interest.

Likely the math question was not whether the aircraft was capable of flying 900 km/h at 8000 ft, but rather this info was given as context. E.g.: an aircraft flies straight and level at 900 km/h at 8 km above the surface; what is the maximum angular speed of the aircraft as seen from a radar which is placed 50 km off the aircraft's track?

High level and fast, I submit the SR-71.  Low level and fast, I submit the B1-B.

It is possible, but it is quite fast.
900 km/h is 486 knots.
Assuming no wind and a standard atmosphere, 486 knots ground speed is 486 knots true airspeed (TAS), which is equivalent to a calibrated airspeed (CAS) of 334 knots at 8000 meters.
At that altitude, the maximum operating speed (Vmo) is usually the limiting factor; if you are a kilometre higher then the maximum operating mach number (Mmo) will typically become limiting.
334 is above the Vmo of some airliners (they will need a little bit of tailwind to achieve 900 km/h over ground at 8000 meters), but many are capable of achieving it without wind.
Some examples of Vmo:

Boeing 737: 340 kts (350 kts for the Original's)
Airbus A320 family: 350 kts
Boeing 747: 365 kts
Boeing 777: 330 kts
Airbus A330: 330 kts
Airbus A340: 330 kts

@mins I am quite sure the OP doesn't really care about the difference between height and altitude. Nor did he specify the elevation of the terrain or the atmospheric conditions. So assuming ground is at sea level and atmosphere is standard does not seem unreasonable here.

The OP didn't ask about airliners but aircraft.  There are many aircraft capable of 334 (and a lot faster) at 8000.

@mins The OP doesn't state that this question was asked in their maths textbook. They stated a question was asked in their math book, which may be a different question. It may not even be related at all to the question posed here, although there likely is a relation otherwise it doesn't make sense to mention the math textbook. Anyway, it doesn't really matter to the answer of the question how the question came about.

@DeltaLima gets the Pendant O' The Day™ award!

@FreeMan Thank you for the award, although I am not sure why. Is it for hanging around here? Or did you mean pedant? In that case I am honoured to beat mins at that category. I'll get my coat...

@ymb1, I only see now that you changed classic to originals. Thanks for that, the text was meant to read that originally (no pun intended :-)

@DeltaLima - you know, it's Friday, I'll give you one of each!

Sure can. Military fighters can exceed 800 KIAS at sea level in full power. Depending upon the aircraft in question, 900 km/h is not an unreasonable speed for a high performance aircraft to fly at while straight and level at that altitude.


text:
What is Pack Flow?
I have heard of something in the Air Conditioning panel called pack flow. I heard that it adjust the flow of pack and Lo is 85% NORM 100% and Hi is like 110% or something like that but I just don't understand what is it adjusting. It is the pressure? I have also heard of ECON FLOW, I heard it saves air correct?

It adjusts the quantity of air (per unit of time = flow) going into the cabin.

It is part of the cooling system. It controls the quantity of the air that goes from pneumatic system into the air conditioning unit - simplified version


text:
Why does the Trent XWB have intermediate compressor stages?
I realized that the Rolls-Royce Trent XWB on the Airbus A350 has intermediate compressor stages. Why is that?
Other engines like the CFM56 or IAE V2500 that power most of the A320-family have low pressure compressor stages (including the fan), and not intermediate ones. And the Trent XWB doesn't have a low pressure compressor. Unless the fan counts as one.
Is the low pressure compressor just the fan on the A350 and they don't call it a fan?

Contains some information: Is it possible to drive a turboprop directly from a jet engine without a gearbox?

Jet engine designers have to find compromises all the time, and the one that counts here is between lower complexity and higher efficiency.
In the 1960s, Rolls-Royce fell behind in large commercial jet engines as competitors introduced large bypass ratio fans such as the GE CF6 and the P&W JT9D, which powered the new generation of wide body jets being introduced at that time. All efforts on the British side came to nought, partially for lack of funding (source: Wikipedia):

The company [Rolls-Royce] went ahead with the project and under the
  leadership of (Adrian) Lombard built the twin spool demonstrator.
  Overall development costs of the RB.178 was £2.6 million. As a
  comparison, the G.E. and P&W companies were awarded nearly $20 million
  by the U.S. DoD ( Department of Defense) to develop and build the TF39
  and STF200 technology demonstrators.

or for the preference of US aircraft manufacturers for US engines. Wikipedia again:

There are perhaps many reasons why the RB.178 failed to take off and
  chief among them was the refusal of Boeing to accept the Rolls-Royce
  engine on the transatlantic 747.

Rolls-Royce realised that they had no chance to catch up by designing yet another two-spool fan and risked the company on a three-spool design, which would later become the RB 211. Wikipedia again:

Meanwhile, Rolls-Royce was also working on a series of triple-spool
  designs as replacements for the Conway, which promised to deliver
  higher efficiencies. In this configuration, three groups of turbines
  spin three separate concentric shafts to power three sections of the
  compressor area running at different speeds. In addition to allowing
  each stage of the compressor to run at its optimal speed, the
  triple-spool design is also more compact and rigid, although more
  complex to build and maintain.

The advantages of using three spools is that the speed of each stage can be more finely optimized, so each stage operates at a higher efficiency. Another promised advantage was the use of carbon fiber for the first stage fan, which allowed considerable weight savings and offered Lockheed a distinct advantage in its L-1011 TriStar over the very similar Douglas DC-10.
However, the carbon fiber fan failed the bird strike test and the engine did not meet its development targets. In the end, Rolls-Royce had to be taken over by the British government and the TriStar entered the market with a one-year delay. But the RB211 went into production, albeit with a delay. The gamble did not pay off commercially, but technically it placed Rolls-Royce instantly ahead of its competitors. When a higher-performance version of the RB211 was added as an engine option to the Boeing 747, the direct comparison with the two-spool designs made the efficiency gains of the three-spool design obvious. FlightGlobal reported in 1980:

The importance placed on fuel saving by airlines is emphasised by
  Qantas' adoption of RB.211-524 power for its new Boeing 747s - the
  only aircraft on which all big three fans are available. Qantas found
  that British Airways' Boeing 747s fitted with RB.211s burnt roughly 7
  per cent less fuel than its JT9D-equipped fleet, a saving of about $1
  million a year at today's prices.

The second oil crisis in 1979-1980 made those efficiency gains very relevant to the airline industry. But the three-spool design offers another advantage: By tailoring the number of stages and their diameter to the desired performance, the same basic design allowed Rolls-Royce to cover a much wider thrust spectrum with one basic engine design. The RR Trent is a direct development of the RB211 and covers a thrust range from 240 to 430 kN without compromising on efficiency. This enables Rolls-Royce to adapt the engine optimally to stretched or shortened versions of most airliners, so it is in an ideal position to offer the most efficient engine for the job. In fact, the successful development of a three-spool fan engine in combination with a better business model helped Rolls-Royce to gain an overall market share of 40%. Wikipedia again:

Sales of the Trent family of engines have made Rolls-Royce the second
  biggest supplier of large civil turbofans after General Electric,
  relegating rival Pratt & Whitney to third position.

That is the reason for the intermediate compressor on the A350: It helps to give it the most efficient engine for the lowest development cost and risk. And yes, the fan is all what is left of the low pressure compressor stage on the Trent 1000. This way it can be run at a lower speed while the intermediate compressor can be built smaller since it runs at a higher speed.

For engines with thrust range as in the A320, the optimum architecture is a two-shaft layout: the cross-sectional area of the compressor reduces, causing the compressor blades to become smaller. If the smaller blades at the end of the compressor can rotate faster, efficiency increases - therefore the engine stages are split in a low pressure and a high pressure section. The fan is simply the first stage of the low pressure compressor: the blade length of the fan allows for spinning at the same rotational speed as the LP compressor and turbine.

The Trent is a larger engine delivering more thrust though, and this justifies adding another shaft. The fan is a separate first stage compressor also delivering thrust. From Rolls Royce, The Jet Engine issue 5:

Larger turbofans can benefit from three shafts: in this configuration, there is a fan (LP), an intermediate (IP) compressor, and an HP compressor all running on separate shafts connected to respective LP, IP, and HP turbines. The separation of the fan and first compressor stages allows the shaft speeds and thus fan and blade velocities to be optimised more closely to the ideal operating conditions of each stage.
The three-shaft layout adds a level of mechanical complexity to the overall engine layout but reduces the reliance on variable geometry compressor features. The main benefit is that high thrust can be developed from a shorter, lighter engine than an equivalently rated two-shaft layout.

Although I don't disagree with the existing answers, I would frame it in a different perspective. The existing answers imply a tradeoff between complexity and efficiency in 2 spool vs 3 spool.  To me that is not the right way to look at it.  For example, this table, show that the 2 spool PW2037 for the 757-200 actually has slightly better thrust specific fuel consumption than the 3 spool RB211-535E for the same plane. Although these are older engines, it does show that 2 spool engines are not always less efficient than 3 spool engines.
To me, a better way to view 2 spools versus 3 spools is tradeoff between aerodynamic complexity and mechanical complexity.
As has already been pointed out, 3 spools allows the 3 different shafts to spin at their own best rates. This is definitely better for aerodynamics (and not just efficiency, but also better stall margin). So if this is so great, why not go to 4 shafts? Or 5 shafts?  Why not have every single compressor stage on it's own shaft?  The answer is that additional shafts add mechanical complexity:

Shafts have to be supported by bearings, so 3 shafts is more bearings
bearings have to be supported by frames, so 3 shafts you need more frames
A typical 2 shaft engine is 2 or 3 frames and 4-6 bearings. A typical 3 shaft engine is 3 or 4 frames and 6 - 8 bearings.

All that extra stuff adds a lot more complexity to the mechanical design.  So, if you go the 2 shaft route, the engine is significantly mechanically simpler.  But the price you have to pay is aerodynamic complexity. To get to a comparable overall efficiency, you have to do a lot more work on the aerodynamic side to get there. For example, you need a lot more variable geometry (variable stator vanes, transient bleed valves, etc), you'll need to do a lot more complex CFD to get every last fraction of a point of efficiency out of each component, etc.

They seem to be different TSFCs for different cruise thrust settings: PW2037 is listed at 6,500 lbf, RB211 at 8,495 lbf.